175 
the compounds of chromium. 
12"5 grains of crystallized chromate of potash (containing 
6'5 grains of chromic acid) were dissolved in a small quan¬ 
tity of boiling-hot distilled water. 52*125 grains of protosul¬ 
phate of iron, recently crystallized, transparent, and of a 
light green colour, were dissolved in boiling-hot distilled 
water, after it had been kept boiling briskly for several 
minutes, in order to free it from air as completely as possible. 
These solutions being mixed, an abundant bulF-coloured pre¬ 
cipitate fell. It was separated by the filter, well washed and 
dried. The weight as dried on the filter was 25 grains, but 
by ignition it was reduced to i6*8i grains. It was a black 
shining powder, not magnetic, and similar in appearance to 
native chromiron ore^ as it is called. 
The residual liquid was neutral, and contained both per¬ 
oxide of iron, and green oxide of chromium. It was diluted 
with water, and then mixed with benzoate of ammonia. The 
object of the dilution was to prevent any benzoate of chro¬ 
mium from being precipitated along with the benzoate of 
iron. The perbenzoate of iron, after being washed and dried, 
was found to contain 1*37 grain of per oxide of iron. The 
green oxide of chromium having been thrown down by an 
alkaline carbonate, washed, dried, and ignited, weighed i*66 
grains. It did not glow, and assumed a blackish colour. 
From this experiment we see, that three atoms of protoxide 
of iron become peroxide when they are employed to reduce 
one atom of chromic acid to green oxide. But an atom of 
chromic acid weighs 6*5, and 3 atoms of protoxide of iron 
require 1*5 atom of oxygen to convert them into peroxide. 
Consequently, chromic acid is a compound of 
